Harbor porpoises as seen from the Golden Gate Bridge. (Photo: William Keener/Golden Gate Cetacean Research)

Harbor porpoises haven’t been seen in San Francisco Bay for more than 60 years. But now, they’re coming back through the Golden Gate in growing numbers and researchers are trying to understand why they’re returning.

The best place to look for them is 220 feet above the water on the pedestrian walkway across the Golden Gate Bridge. That’s where Bill Keener of Golden Gate Cetacean Research photographs them, holding a massive telephoto lens over the side of the railing.

“There’s a porpoise right there, coming very, very close,” he says pointing. A dark shape appears in the water. It’s a harbor porpoise, coming up for air. “And here’s a mother and calf coming straight at us.”

The porpoises have dark gray backs and pale bellies. They’re about five feet long, smaller than most of their dolphin relatives.

“Look at that! That one’s on its side,” Keener says. “The porpoise turned on its side. It’s spinning and it’s feeding.”

Porpoises spin as they go after schools of herring and anchovies, which means these porpoises are feeding in the middle of a heavily-trafficked shipping lane. “The porpoises have found a way to not only avoid the ships, but it’s also the noise they make,” says Keener.

Studying a Shy Marine Mammal

Seeing this behavior is huge for Keener. Harbor porpoises are notoriously shy in the open ocean, so they’re tough for researchers to study. Here in the bay, Keener and his colleagues have identified 250 individual porpoises with their photos by looking for unique scars and color patterns on the animals.

Porpoise mating display as seen from the bridge. (Photo: William Keener/Golden Gate Cetacean Research)

When these researchers first started their work on the bridge, they caused a bit of a stir. “You noticed there was a Golden Gate Bridge patrol officer here just a few minutes ago,” says Keener. “Well, we’re staring down at the water for hours. They’d start getting worried about us. But they know us now. They know what we’re doing.”

Of course, the big question is why harbor porpoises disappeared in the first place. Keener says the bay has historically been porpoise habitat. Their bones have been found from hundreds of years ago.

“And then there were reports in the 1930s. And then we don’t really have reports from around World War II. And there were a lot of things going on during World War II that could have caused that.”

San Francisco Bay became a wartime port and a major ship-building center. The Navy strung a seven-mile-long net underwater across the opening of the bay to keep out Japanese submarines. Hundreds of mines were planted in the waters outside the Golden Gate.

Keener says all that activity certainly would have disturbed the porpoises. But there’s a bigger change that may have driven them away.

The Changing Bay

The best time to spot harbor porpoises from the Golden Gate Bridge walkway is an hour or two prior to a high tide. Check out a tide table to time your visit and report your sightings online.

To see it, we head toward the Golden Gate Bridge on a twenty-two foot boat with Jonathan Stern, a whale researcher at San Francisco State University. Stern was the first person to spot the porpoises in the bay three years ago.

“I just couldn’t figure out what they were doing here. It’s like when you see somebody you’re used to seeing at work and you see them somewhere, in Hawaii or something. What are you doing here? You’re out of place,” says Stern.

The bay we’re gliding over today is a far cry from the bay in the 1950s and 60s. As the Bay Area boomed, so did water pollution. Keener says raw sewage used to flow right into the bay. “I remember coming across the Bay Bridge when I was very young and it would just smell. It would stink.”

After the Clean Water Act was passed in 1972, the bay’s water quality began to improve. But Stern says it took time for the food web to come back. “It takes the biology a while to track the chemistry. So it’s not surprising that it’s taken years for this ecosystem to generate like this.”

Stern says it’s also possible that the porpoises had to rediscover the bay. “Because over 60 years, we’re talking about a number of generations of porpoises. So it’s quite likely that San Francisco Bay as a location, as a habitat was out of the institutional memory of the harbor porpoises off the coast here.”

Bill Keener and Jonathan Stern search for porpoises in the bay.

Bill Keener and Jonathan Stern search for porpoises in the bay.

As we slow down under the bridge’s span, Keener keeps an eye out. “There are porpoises between us and the south tower at 200 yards,” he says.

Keener and Stern have a special permit that allows them to approach the porpoises. We wait, listening for them to surface.

“I just heard one here. Here’s a cow-calf pair close to the boat and we’ll hear this puff,” Keener says.

We hear two loud puffs as the porpoises surface just off the bow. “The old time sailors used to call them puffing pigs. That’s the exhalation,” says Keener.

The harbor porpoises seem calm around boats in the bay, which Stern says will let researchers study their life cycle and social structure, as well as how they might react to big events like the upcoming America’s Cup race. Overall, he says it’s a good sign that the porpoises are here.

“It’s one of those very few good news environmental stories. And it’s in our backyard. You know, there was the will to get the bay cleaner and we’re now starting to see the effects of that. It gives one hope.”

Author

Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs - all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR's Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.